The human coagulation system is able to stop bleeding and initiate healing. The function of the system is well known and extensively investigated. However, the importance of coagulation products in the initiation of healing has only been recognized recently.
Blood products, such as fibrin sealants and platelets concentrates, are produced by isolating the platelet rich plasma (PRP) from anti-coagulated whole blood. The presence of platelets and plasma partly imitates the natural human coagulation system upon thrombin activation. This leads to a platelet containing autologous concentrate of growth promoting factors in a fibrin matrix. Such a composition can be used for covering wound surfaces and is claimed to initiate healing.
“Platelet-rich fibrin (PRF): A second-generation platelet concentrate. Part I: Technological concepts and evolution, Oral Surg Oral Med Oral Pathol Oral Radiol Ended 2006; 101:E37-44” by David M. Dohan et al describes how to prepare a platelet rich solid fibrin network from whole blood without adding any additives or reagents. The PRF protocol is: A blood sample is taken without anticoagulant in 10-mL glass tubes or glass coated plastic which are immediately centrifuged at approximately 400 g for 10 minutes. The absence of anticoagulant implies the activation in a few minutes of most platelets of the blood sample in contact with the glass tube walls and the release of the coagulation cascades. Fibrinogen is initially concentrated in the top part of the tube, before the circulating thrombin transforms it into fibrin. A fibrin clot is then obtained in the middle of the tube, extending from the upper part of the red corpuscles at the bottom of the tube to the cellular plasma at the top. Platelets are trapped massively in the fibrin meshes. The success of this technique entirely depends on the speed of blood collection and transfer to the centrifuge. Indeed, without anticoagulant, the blood samples start to coagulate almost immediately upon contact with the tube glass, and it takes a minimum of a few minutes of centrifugation to concentrate fibrinogen in the middle and upper part of the tube. Quick handling is the only way to obtain a clinically usable PRF clot. If the duration required to collect blood and launch centrifugation is overly long, failure will occur: The fibrin will polymerize in a diffuse way in the tube and only a small blood clot without consistency will be obtained. In conclusion, the PRF protocol makes it possible to collect a fibrin clot charged with serum and platelets. By removing the clot from the tube, manually cutting of the red cells part, and manually driving out the fluids trapped in the fibrin matrix (serum), practitioners will obtain autologous fibrin membranes.
However this fibrin network includes a red thrombus containing a substantial part of red blood cells, which have to be manually cut off. Furthermore the components of the produced fibrin network, such as fibrin, leukocytes and thrombocytes, are arbitrary distributed and enmeshed within the product. The recovery of leukocytes are not described and at the low g force used, the recovery of leukocytes is low as some will be located in the red cell part. The enmeshment of cells within the fibrin leads to absent or slow release of these cells and thereby inhibits the contact-dependent anti-microbicidal potential of the included leucocytes.
“Oral Surg Oral Med Oral Pathol Oral Radial Endod 2006; 101:E37-44 and 2006; 101:E45-50” by Dohan et al describes a network that does not represent a platelet concentrate in a shape and structure, which is directly applicable for covering wound surfaces. To obtain a shape and form/rigidity useable for covering wound surfaces and prevent red blood cell inclusion, the known platelet rich solid fibrin network will have to be reshaped manually and compressed. Furthermore, the method comprises several steps and cannot be prepared in one closed system and is therefore not convenient for clinical use.
“Cell separation in the buffy coat. Biorheology. 1988; 25 (4):663-73” by Sutton et al describes how anti-coagulated full blood will separate into several layers upon centrifugation or passive sedimentation; Red blood cells, leukocytes and platelets (=buffycoat) and plasma. Further, by using centrifugation force of 10000 g for 10 minutes and using a float of density of 1.053, the buffy coat can be fixed by Glutaraldehyde, and removed for investigation; however, this cannot be used clinically due to the toxicity of the Glutaraldehyde. Several methods for extracting the buffycoat from anti-coagulated blood exists including the use of density defined substances (ie. Lymphoprep). In addition to the need for anti-coagulated blood the extracted cells will be suspended—and mixed (disorganized)—in the plasma that inevitably will be included.
EP 1637145A describes a method of filtration of cells from a suspension (eg. blood cells including platelets and leukocytes) through a sheet like porous membrane, leaving the cells in the membrane as described. The sheet porous material can be prepared from fibrin. However, no layered structure is obtained, the cell are trapped in depth in the porous material and the use of allogeneic fibrin raises the risk of cross infection from other humans. Furthermore, the method comprises several steps and cannot be prepared in one closed system and is therefore not convenient for clinical use.
Known methods are limited in their use, especially clinical use. The addition of anti-coagulants prior to cell separation lead to products not completely autologous, furthermore the release of substances promoting wound healing (e.g. growth factors) requires mixing with other non-autologous substances (e.g. thrombin, Ca2+ etc.) leading to homogenous final products without the desired distribution of cells.
Known methods excluding anti-coagulation lead to a disorganized distribution of cells, the cells are locked inside the product, effectively limiting the release and potential of these cells. Furthermore these methods need manual handling outside a closed system to obtain a product physically suitable for clinical use, an inadequately defined handling that leads to a variable outcome with a lower than optimal cell yield. Furthermore, the manual handling will require labor time (cost) and prolong the preparation time.
Methods describing a well defined layered structure depend on anti-coagulation and addition of toxic components, not suitable for clinical use, for the fixation and self-sustainability of the structure obtained.